This disclosure pertains to X-ray optical systems, encompassing such systems as used in X-ray telescopes, X-ray laser systems, and X-ray microlithography systems. More specifically, the disclosure pertains to multilayer-coated mirrors as used in X-ray optical systems. By xe2x80x9cX-rayxe2x80x9d is meant not only the so-called xe2x80x9chardxe2x80x9d X-ray wavelengths of about 1 nm or less, but also the so-called xe2x80x9csoftxe2x80x9d X-ray wavelengths of about 20 nm to about 1 nm.
The complex refractive index of a substance is expressed by Equation (1), below. Since both xcex4 and k are substantially less than unity within the wavelength range encompassed by the X-ray wavelengths, reflective-optical systems are used in X-ray optical systems.
n=1xe2x88x92xcex4xe2x88x92ixc2x7kxe2x80x83xe2x80x83(1)
In the case of oblique-incidence optical systems exploiting full X-ray reflection, the critical angle of total reflection, xcex8c is extremely small. As a result, reflectivity is extremely small at angles of incidence near perpendicular. Hence, multilayer-coated mirrors are used in reflective optical systems for use with X-ray wavelengths. Multilayer-coated mirrors are made by alternatingly laminating two types of substances on the surface of a mirror substrate. The substances are selected so as to exhibit high-amplitude reflectivity at the boundary. The thickness of each of the layers is established based on light-interference theory such that the waves of X-ray light reflected by the respective layer boundaries surfaces interfere constructively. With respect to one of the selected substances, the difference between its refractive index in the X-ray wavelength range in which the mirror is to be used and its refractive index in a vacuum (n=1) is small. The other substance is one in which this difference is large.
Since multilayer-coated mirrors are capable of reflecting perpendicularly incident X-rays, such mirrors can be used in X-ray optical systems that employ perpendicular reflection to yield lower aberrations than exhibited by oblique-incidence optical systems relying on total reflection. Also, a multilayer-coated mirror exhibits wavelength selectivity by strongly reflecting X-rays whenever Bragg""s condition, expressed by Equation (2), is met.
2dxc2x7sin xcex8=mxc2x7xcexxe2x80x83xe2x80x83(2)
In Equation (2), d is the period length of the multilayer coating, xcex8 is the oblique angle of incidence, xcex is the X-ray wavelength, and m is the order.
A well-known example of a multilayer coating is a W/C multilayer coating, in which tungsten (W) and carbon (C) are alternatingly laminated on the mirror substrate. Another well-known example is a Mo/C multilayer coating, in which molybdenum (Mo) and carbon are alternatingly laminated on the mirror substrate. These multilayer coatings are formed by thin-film-formation techniques, such as sputtering, vacuum deposition, or chemical vapor deposition (CVD).
Among several types of multilayer coatings that could be used for the soft X-ray wavelengths, Mo/Si multilayer coatings exhibit high reflectivity at the long L-absorption end of the wavelength spectrum of Si (12.6 nm wavelength). Thus, multilayer coatings easily can be produced that exhibit greater than 60% reflectivity (at perpendicular incidence) to soft-X-ray wavelengths of approximately 13 nm. Reflective mirrors having Mo/Si multilayer coatings currently are being used in research applications such as X-ray telescopes and X-ray laser resonators, and these mirrors also are being actively considered for use in reduction-projection microlithography systems that use soft X-rays. These soft X-rays also are termed xe2x80x9cextreme ultravioletxe2x80x9d (EUV) light, and microlithography systems utilizing this wavelength range of light are called extreme ultraviolet lithography (EUVL) systems.
Whereas high-reflectivity Mo/Si multilayer-coated mirrors can be produced by sputtering, thin films formed by sputtering generally are subject to internal compressive stress (Sey-Shing Sun: J. Vac. Sci. Technol. A4(3), May/June 1986). As a result, in multilayer-film mirrors made using this technique, problems arise due to the internal stress that develops in the Mo/Si multilayer coating. This stress causes deformation of the mirror substrate, which produces wavefront aberrations in the X-ray optical system and corresponding decreases in optical performance of the mirror.
In view of the shortcomings of the prior art as summarized above, the present invention provides, inter alia, multilayer-coated mirrors that exhibit decreased internal stress in the multilayer coatings, thereby providing multilayer-coated mirrors exhibiting better optical performance than conventional multilayer-film mirrors.
According to a first aspect of the invention, multilayer-coated mirrors are provided. A representative embodiment of such a mirror comprises a mirror substrate and a multilayer coating formed on the mirror substrate. The multilayer coating comprises multiple layer units each comprising a respective molybdenum (Mo) layer and a respective silicon (Si) layer. The layer units are formed such that, in the multilayer coating, the Mo layers alternate with the Si layers. At least one of the Mo layers adjacent a respective Si layer includes a molybdenum oxide layer at an interface surface with the adjacent respective Si layer. Desirably, the molybdenum oxide layer is on an interface surface of the Mo layer that faces away from the mirror substrate. Also, the molybdenum oxide layer desirably has a thickness of at least 0.5 nm.
According to another aspect of the invention, methods are provided for manufacturing a multilayer-coated mirror. In an exemplary embodiment of such a method, multiple Mo/Si layer units are formed on a surface of a mirror substrate. Each layer unit comprises a respective Mo layer and a respective Si layer. The layer sets are formed such that, in the multilayer coating, the Mo layers alternate with the Si layers. After forming at least one of the Mo layers, oxygen ions are added to the Mo layer before forming a Si layer over the Mo layer. The oxygen ions can be added by irradiating the Mo layer with oxygen ions. Alternatively, the oxygen ions can be added by providing oxygen gas or ozone gas, producing oxygen ions from said gas, and exposing the Mo layer to the oxygen ions.
In another method embodiment, on a surface of a mirror substrate multiple Mo/Si layer units are formed each comprising a respective Mo layer and a respective Si layer. The layer units are formed such that, in the multilayer coating, the Mo layers alternate with the Si layers. After forming at least one of the Mo layers, an oxygen-containing boundary layer is formed on the Mo layer in presence of an atmosphere comprising oxygen.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description and the accompanying drawing.